The present invention relates to an exhaust gas purification system having a particulate filter used for after-treatment of an exhaust gas mainly of a diesel engine and more specifically to a technology for dealing with ash accumulated on a particulate filter. The present invention further relates to an exhaust gas purification method for an internal combustion engine.
A diesel particulate filter is a particulate trapping device consisting of a honeycomb monolith made of ceramic or the like and generally used for removing particulate matters (hereinafter referred to as particulate) emitted from a diesel engine. During operation of the engine, the particulate is accumulated on the diesel particulate filter increasingly. At some time when the accumulated amount of particulate exceeds an allowable amount, the diesel particulate filter is clogged to cause the exhaust pressure to become higher, thus causing a bad influence on the performance efficiency of the engine. For this reason, the accumulated particulate is burnt periodically so as to prevent clogging of the particulate filter.
Particulate is a general term for particulate matters in the exhaust gas and contains incombustible matters other than combustible matters such as soot. Representative one of the incombustible matters is calcium sulfate that is a product of burnt additives of engine oil. The incombustible matters such as calcium sulfate are emitted from the engine as particulate components and accumulated on the diesel particulate filter. Differing from other particulate matters such as soot, the incombustible matters cannot be removed by regeneration treatment that is carried out with a view to burning the particulate but remains on the diesel particulate filter as powdery residue (hereinafter referred to as ash) after regeneration. Accumulation of an excess amount of ash on the diesel particulate filter not only causes a rise of exhaust pressure but a decrease in the effective filtration area and therefore a decrease in the ability of the diesel particulate filter. Thus, when the accumulated amount of particulate exceeds an allowable value, it becomes necessary to take a particular step such as a step for replacement of the diesel particulate filter.
In this connection, as a measure against ash there is known such a method of assuming that accumulation of a predetermined amount of ash on a diesel particulate filter occurs per a predetermined mileage and removing the particulate from the diesel particulate filter each time of running of a vehicle over the predetermined mileage as disclosed in Unexamined Japanese Patent Publication No. 2000-234509.